Alkali-Metal Thiogermanates: Sodium Channels and Variations on the La3CuSiS7 Structure Type


Five new isotypic quaternary chalcogenides containing rare-earth metal atoms crystallizing in the hexagonal noncentrosymmetric space group P63 (No. 173) with the La3CuSiS7 structure type have been synthesized by reacting the appropriate anhydrous rare-earth trichloride with sodium thiogermanate, Na2GeS3. The reaction between LnCl3 and Na2GeS3 in an evacuated fused-silica ampule produced high yields of good-quality crystals of NaLn3GeS7 [Ln = Ce (I), Nd (II), Sm (III), Gd (IV), and Yb (V)], while a similar reaction between EuCl3 and Na2GeS3 yielded a quinary chloride thiogermanate, Na1.2Eu3.4Cl2Ge3S9 (VI), incorporating a cyclic trimeric Ge3S9 building unit and adopting a structure related to La3CuSiS7. The crystal structure of the compounds comprises a complex network of bicapped trigonal-prismatic LnS8 and GeS4 tetrahedra, which creates channels along the [001] direction. The Na+ cations reside in these channels within trigonally distorted octahedral coordination environments, surrounded by six S atoms. For compounds III-V, the temperature dependence of the magnetic susceptibility indicates that these compounds are paramagnetic with μeff. = 1.86, 8.01, and 3.87 μB, for III-V, respectively. The experimental μeff for IV is close to the theoretical value of 7.94 for free Gd3+ ions, while μeff values for III and V deviate from their theoretical values of 0.86 and 4.54 μB for Sm3+ and Yb3+ ions, respectively. These compounds are semiconductors with optical band gaps of around 1.3 eV for III and V. Extended Hückel calculations suggest that the valence band comprises primarily S 3p and the bottom of the conduction band is dominated by empty rare-earth 5d orbitals. Compound VI exhibits a sharp optical absorption of around 2.18 eV, which is attributed to the f → d transition of EuII. The effective magnetic moment of 7.94 μB/Eu is in excellent agreement with the theoretical value of 7.94 μB for the free Eu2+ ion.



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